An organic electroluminescence (EL) device includes a fluorescent organic EL device or a phosphorescent organic EL device, and a device design optimum for the emission mechanism of each type of organic EL device has been studied. It is known that a highly efficient phosphorescent organic EL device cannot be obtained by merely applying fluorescent device technology due to the emission characteristics. The reasons therefor are generally considered to be as follows.
Specifically, since phosphorescence utilizes triplet excitons, a compound used for forming an emitting layer must have a large energy gap. This is because the energy gap (hereinafter often referred to as “singlet energy”) of a compound is normally larger than the triplet energy (in the invention, the difference in energy between the lowest excited triplet state and the ground state) of the compound.
In order to confine the triplet energy of a phosphorescent dopant material efficiently in an emitting layer, it is required to use, in an emitting layer, a host material having a triplet energy larger than that of the phosphorescent dopant material.
In order to reduce the driving voltage of an organic EL device, it is necessary to use a material having excellent carrier-injecting properties and carrier-transporting properties. However, when a material having excellent carrier-injecting properties and carrier-transporting properties is used, although the driving voltage is not lowered, but the carrier balance in an emitting layer is deteriorated, leading to shortening of the device life. That is, a carrier-transporting material that reduces the driving voltage while maintaining the life of the device is required.
It is thus necessary to select materials and a device design differing from those of the fluorescent organic EL device in order to obtain a highly efficient phosphorescent organic EL device.
Research on the materials has been extensively made, and some reports were made (Patent Documents 1 to 3).
In Patent Document 1 and Patent Document 2, a benzimidazole compound is exemplified and used as an electron-transporting layer material. However, since the triplet energy of a host material of an emitting layer which is used in combination is small, and the efficiency is low. In addition, a host material to be combined has a structure that does not contain a dibenzofuran ring or a dibenzothiophene ring, carrier injection/transportation properties is low, and hence, the driving voltage tends to be high.
Patent Document 3 exemplifies a benzimidazole compound, which is used as an electron-transporting layer material. The triplet energy thereof is small, and as a result, the energy of an emitting layer is leaked to the electron-transporting layer side, leading to a lowering of the efficiency.